1. Field of the Invention:
The present invention relates to improvements in a developer layer forming apparatus for regulating the thickness of a developer layer which is being held on and carried by a developer carrying member in developing means for developing a latent image on an image retainer especially for electrophotography.
2. Description of the Prior Art:
Generally speaking, when a latent image on an image retainer is to be developed with a developer held on and carried by its carrying member, the quality of the image obtained can be improved with a high contrast by reducing the gap between a latent image retainer (which may be referred simply to an "image retainer") and the developer carrying member. If the gap between the image retainer and the developer carrying member is reduced, the contact developing method, in which the image retainer and the developer layer is held in contact for the development, should reduce the amount of the developer, which is to be held on and carried by the developer carrying member, to a small proper range so that the developer may neither be tamped between the image retainer and the developer carrying member nor be broken the latent image or the developed image by the tamping pressure.
Incidentally, the "amount of developer" will be herein defined as the weight of the developer per unit area, which is held on the surface of the developer carrying member.
Moreover, the non-contact developing method, in which the image retainer and the developer layer is held in non-contact state for the development, should also reduce the amount of developer to a small proper range so that the image retainer and the developer layer may not come into contact when the gap between the image retainer and the developer carrying member is reduced.
In the developing method using a two-component developer according to the prior art, for example, the development has been carried out using a developing device having the following structure. A non-magnetic cylindrical member or a non-magnetic belt is used as the developer carrying member, and a magnet is arranged at the opposite side to the developer carrying surface. The arrangement of this magnet may be fixed or movable, i.e., rotatable on a certain axis so long as it is suitable the developing conditions intended.
The magnetic carrier of the developer on the developing carrying member is subjected to the magnetic force of the magnet arranged at the opposite side to the developer carrying member so that it is erected in the form of ear in accordance with the magnetic force of the magnet and the magnetism of the carrier and held on the developer carrying member. On the other hand, the non-magnetic toner is frictionally charged by the friction with the magnetic carrier as a result that it is agitated in the developer reservoir so that the non-magnetic toner and the magnetic carrier acquire charges of opposite polarities and attached to each other by the coulomb force.
Since the magnetic carrier is held on the developer carrying member by the magnetic field of the magnet, too, the non-magnetic toner is also held together with the magnetic carrier on the developer carrying member so that a developer layer composed of the magnetic carrier and the non-magnetic toner is formed on the developer carrying member.
This developer layer is carried to and held on the developer carrying member to develop a latent image on the image retainer.
This developer layer must satisfy the aforementioned minimum conditions that it should neither be tamped in the gap between the image retainer or the developer carrying member nor come into contact with the image retainer. In addition, the developer layer must satisfy the following conditions:
(1) The "fog" phenomenon, in which the toner deposits on the surface of the image retainer other than an image portion, should not appear; PA1 (2) The "carrier deposition" phenomenon, in which the carrier deposits on the image retainer surface, should not appear; PA1 (3) The toner in an amount enough for a sufficient image density should deposit on a latent image portion; and PA1 (4) The toner image having deposited should have a gradation capable of reproducing the gradation of the latent image. In order to satisfy these condition, the
following items should be within proper ranges:
the physical properties and agitating methods of the developer; the physical properties and relative velocities of the image retainer to the developer layer; means for forming a latent image on the image retainer; the electric biases applied between the image retainer and the developer carrying member; and the magnetic force and the shapes of the magnetic fields of the magnet.
The conditions further include the amounts and thickness of the developer layer on the surface of the developer carrying member, which should be within shorter proper ranges than the aforementioned ones.
In order that the developer layer held on the surface of the developer carrying member may have desired amounts and thickness according to the prior art, a layer thickness regulating plate 205 made of metal or resin is arranged in the vicinity of the surface of a developer carrying member 203, as shown in FIG. 3, so that a developer D may be held on and carried by the surface of the developer carrying member 203 to a developing region through the gap between the layer thickness regulating plate 205 and the surface of the developer carrying member 203.
The developer carrying member 203 is made of a non-magnetic cylinder having a magnet 204 therein, as shown.
In order that the layer thickness of the developer D may be reduced to about 1 mm or less by the method using that layer thickness regulating plate 205, the gap between this plate 205 and the surface of the developer carrying member 203 should be about 0.5 mm or less. The reason for this dimensional restriction will be explained in the following. When the developer D passes through the gap between the layer thickness regulating plate 205 and the developer carrying member 203, it is constricted into a developer layer of high density. After having passed through the gap between the layer thickness regulating plate 205 and the developer carrying member 203, on the contrary, the developer D on its carrying member 203 is erected by the magnetic force of the magnet 204 so that it becomes a developer layer having a lower density. This layer is 1.5 to 3 times as high as that which is passing through the gap between the layer thickness regulating plate 205 and the developer carrying member 203.
An ordinary copying machine or printer for developing and visualizing an electrostatic latent image has an image width of several hundreds mm.
Highly accurate machining and adjustment of the layer thickness regulating plate are not easy but required for evenly and stably setting at about 0.5 mm or less the aforementioned gap between the layer thickness regulating plate and the developer carrying member all over the width of the image.
With this in mind, therefore, we have invented and applied for patent (as Japanese patent application Nos. 34318/1986 and 34319/1986) a method of forming a developer layer by making the use of the fact that a gap sized substantially equal to the particle diameter of a magnetic carrier can be formed between an elastic plate and a developer carrying member, when the developer sneaks between the elastic plate and the developer carrying member, by pushing the elastic plate onto the surface of the developer carrying member.
FIG. 4 is a diagram showing an example of the means for carrying out the method disclosed in the above-specified Patent Applications.
The elastic plate is forced onto the surface of the developer carrying member such that its leading end is directed to the upstream of the developer carrying member. As shown, the developer carrying member 103 is made of a non-magnetic cylinder, and the elastic plate 105 is flattened and made of a metal such as phosphor bronze or stainless steel. If this elastic plate 105 is forced onto the surface of the developer carrying member 103, a wedge-shaped space 110 is formed between the leading end of the elastic plate 105 and the forced contact. If, in this state, the developer carrying member 103 is moved in a predetermined direction, the developer D held on the developer carrying member 103 by the magnetic field of the magnet is partially allowed to enter the wedge-shaped shaped space 110 and partially not allowed but carried to the opposite surface of the elastic plate 105 to the developer carrying member 103. Only the developer D having entered into the wedge-shaped space 110 is carried to the developing region through the gap between the developer carrying member 103 and the elastic plate 105 by the frictional force established itself and the developer carrying member 103. At this time, the amount of the developer allowed to pass through the gap between the developer carrying member 103 and the elastic plate 105 is determined by both the height of the opening of the wedge-shaped space 110 and the line pressure for forcing the elastic plate 105 onto the developer carrying member 103. This determination naturally corresponds to the case in which the aforementioned conditions such as the physical properties and agitating method of the developer and the magnetic field and force of the magnet are fixed.
The developer having entered into the wedge-shaped space 110 through its opening is forced in the converging direction of the wedge-shaped space 110 by both the frictional force, which is generated between the magnetic carrier and the developer carrying member 103 as a result that it is forced onto the developer carrying member 103 by the magnetic field of the magnet arranged inside the developer carrying member 103, and the pressure of the developer D which is being sequentially forced into the wedge-shaped space 110. The developer D thus forced pushes the elastic plate 105 to sneak through the gap between the elastic plate 105 and the developer carrying member 103.
If the distance of the gap between the elastic plate 105 and the developer carrying member 103 is longer in the sneaking region than the size of a particle of carrier, what supports the elastic plate 105 is both the vertical component of the force for pushing the developer D from the back with respect to the elastic plate 105 and the force for erecting the carrier along the magnetic field generated by the magnet.
If, on the contrary, the distance of the gap between the elastic plate 105 and the developer carrying member 103 becomes equal to the particle size of the carrier, the strength of the carrier has to supports the elastic plate 105. In order, therefore, that the gap distance between the elastic plate 105 and the developer carrying member 103 may be smaller than the carrier particle size, it is necessary either to block the sneaking itself of the developer D into the gap between the elastic plate 105 and the developer carrying member 103 or to apply a line pressure capable of breaking the carrier itself. The relation of the gap distance between the elastic plate 105 and the developer carrying member 103 to the line pressure is plotted in FIG. 5. As shown, when the gap distance is equal to the carrier particle size, the gap distance is constant over a wide range of the line pressure upon the elastic plate 105 and the developer carrying member 103 to facilitate the setting of the elastic plate 105.
As stated above, by setting the gap between the elastic plate and the developer carrying member equal to a value corresponding to the carrier particle size when the developer is passed therethrough, the thin developer layer can be formed stably and evenly to make the gap between the image retainer and the developer carrying member smaller than that of the prior art so that images of high quality can be formed.
In the method described above, however, the metal plate of phosphor bronze or stainless steel is used as the elastic plate. As a result, when the magnetic carrier is sandwiched between the developer carrying member 103 and the elastic plate 105, as shown in FIG. 6, its contacts are located at one to three points at the respective sides of the elastic plate 105 and the developer carrying member 103 and its contact diameters are one five hundredth to one thousandth as small as the particle size to provide a remarkably small area. This increases the force applied per unit area of each contact point so that the magnetic carrier is heated under a high pressure until it is possibly broken into pieces. During the revolution of the developer carrying member, moreover, the toner is heated to a high temperature and broken into fine pieces in case it is sandwiched together with the magnetic carrier, as shown at the righthand side of FIG. 6. These pieces of the toner have a tendency to aggregate after they have been once melted, because the toner is made of a synthetic resin. The toner and magnetic carrier thus broken into finer powdery state than the normal ones are caught at the leading end portion of the elastic plate 105, as shown in FIG. 7, by the physical attraction, when the molten toner solidifies again, to form a sticking deposition 9 as the number of the copying operations increases. As the amount of this deposition increases the more, the opening area of the wedge-shaped space 110 becomes the more smaller so that a sufficient amount of developer cannot be fed to the developer carrying member 103.
Since, moreover, the amount of this sticking deposition is different in accordance with the delicate changes in the conditions such as the surface properties of the elastic plate or the physical properties of the toner, it becomes not even but various in the individual positions taken in the widthwise direction, as shown in FIG. 8. The resultant problem is that streaks are formed in the development.
An experiment was carried out in order to obtain a time until a uniform layer could not be formed due to the increase of the amount of sticking deposition when the thickness or deformed amount of the elastic plate was reduced so that a pressure applied on and a temperature at the toner or carrier in the vicinity of the pressure contact portion of the elastic plate with the developer carrying member were reduced.
Said deformed amount was expressed by a distance from a point where the elastic plate was in contact with the developer carrying member with a pressing force (line pressure) of zero to a point where the fixing portion of the elastic plate fixed on the elastic plate holding member had been moved in order to obtain a predetermined pressing force.
The elastic plate used in this experiment was a phosphor bronze plate, the carrier was a ferrite core coated with acrylic or styrene resin and the toner was a pigment dispersed into a polyester resin etc.
According to the experiment, following results were obtained.
When a time corresponding to several hundreds copies with a line pressure of 2 g/mm had been lapsed a uniform layer could not be formed, but when the line pressure was reduced to 0.2 g/mm a uniform layer could be formed until a time corresponding to one thousand or two thousands copies had been lapsed, and when the line pressure was reduced to 0.02 g/mm a uniform layer could be formed until a time corresponding to ten thousands copies had been lapsed. However, the phosphor bronze plate used for obtaining the line pressure of 0.02 g/mm, for example, was of 0.05 mm in thickness so that it was deformed permanently by a small pressure from the outside and had to be handled very carefully. Further, the deformed amount of the phosphor bronze plate at the contact state was as small as about 0.1 mm with respect to the width or length of the developer carrying member in the axial direction of about 300 mm, so that a very high precision was required for holding the phosphor bronze plate and the sleeve and the manufacturing became difficult.
Accordingly, it is considered that an effective contact area of the elastic plate with the toner or carrier is made large by using an elastic rubber member as an elastic plate in place of the metalic elastic plate to reduce the effective pressure.
Specifically, it is supporsed that if the elastic rubber member is used as the elastic plate an effective contact area of the elastic plate with the toner or carrier is more increased and a force per unit contact area or an effective pressure becomes smaller than that of the metallic elastic plate, because the elastic rubber member is deformed according to the figure of the toner or carrier even if the pressure applied on the toner or carrier is not varied. Accordingly, it is expected to reduce the possibility of breakage and melt due to the increased temperature and pressure.
In case, however, the elastic rubber member may have its leading end undulating due to the delicate difference in the mounting method, as shown in FIG. 9, in case the rubber member of not more than 1 mm in thickness and large in width is used. With these undulations, a larger amount of developer is fed at the crests of the undulations whereas a smaller amount of developer is fed at the roots. Then, the development will establish such streaks other than the aforementioned ones as correspond to the undulations.